1. Field of the Invention
The present invention relates to ion implantation systems, and particularly to ion implantation systems with an intelligent supervision system using an expert system during ion implantation.
2. Description of the Prior Art
The electrical properties of a semiconductor crystal can be modified by introducing controlled amounts of dopant impurities into the crystal. Ion implantation and diffusion are the most commonly used methods to introduce impurities into a semiconductor wafer. In silicon semiconductor technology, for example, p-type impurities such as boron or BF.sub.2, and n-type impurities such as arsenic, phosphorus, or antimony are typical dopants.
Doping of semiconductor wafers by diffusion is done by introducing impurities into the wafer and redistributing them within the semiconductor crystals at elevated temperature. Unlike diffusion, ion implantation is a low-temperature process in which ionized dopants are accelerated to high energies so that the dopants penetrate to a certain depth when they impact on a target wafer. During the past two decades, ion implantation has become the preferred method of doping semiconductors because of its flexibility in achieving different impurity profiles, and its better control of dopant concentration.
FIG. 1A shows the block diagram of a typical ion implantation machine. The main elements of the machine include an ion source 10, beam transport 12, a target chamber 14 and a man-machine interface 16. The ion source 10 produces a high-density of ions from which the beam transport 12 extracts a focused beam of ions and transports to the target wafer in the target chamber 14.
FIG. 1B shows the schematic of a typical beam transport 12, which includes a mass analyzer 120, an accelerator 124, a focusing system 126, and a scan system 128. The mass analyzer 120 is used to select one of many types of ions from the ion source 10 through the use of a strong magnetic field that separates the ions according to their mass-to-charge ratio. After leaving the mass analyzer 120, the ion beam is accelerated by the accelerator 124 to gain the desired kinetic energy. Next, the accelerated ion beam is focused by the focusing system 126, and then is swept both vertically and horizontally by the scan system 128 across the wafer 140 to distribute the dopants uniformly over the surface of the wafer 140 which is mounted in the target chamber 14. The scan system 128 typically includes X-scan plates 1280 and Y-scan plates 1282.
The man-machine interface 16 (FIG. 1A) is used by an operator to control implantation system parameters, such as recipe, acceleration voltage or ion dosage. This interface 16 can also display other system parameters, such as beam current, on a screen so that the operator can continuously monitor the implantation process. The operator can adjust the implantation process as necessary by controlling, for example, the X-plate and Y-plate voltage by manipulating an adjustment stick, which is commonly referred to as a joystick.
FIG. 2A shows an ion beam map as displayed on the man-machine interface 16 (FIG. 1), which is typically used in supervising an ion implantation process during the fabrication of an integrated circuit. FIGS. 3A to 3H demonstrate the formation of the beam map. Referring to FIG. 3A, ion beam 30 is blocked before time t.sub.1 by the beam defining aperture 32, so that the beam current reaching the wafer 34 remains zero as shown in FIG. 3B. Next, as shown in FIG. 3C, the ion beam 30 begins to sweep onto the wafer 34 at time t.sub.2. Because part of the ion beam 30 contacts the wafer 34, the beam current rises as shown in FIG. 3D. Referring to FIG. 3E, the entire ion beam 30 sweeps across the wafer 34 between time t.sub.2 and t.sub.3, resulting a constant maximum beam current as shown in FIG. 3F. Finally, as the ion beam 30 is blocked again by the other end of the beam defining aperture 32 after t.sub.3 (FIG. 3G), the beam current decreases as shown in FIG. 3H.
Each time the ion beam sweeps across the wafer, the beam map display generates a different trace. The width of the trace is proportional to the length of the beam path across the wafer. FIGS. 4A to 4D illustrate examples of different traces and the corresponding beam path across the wafer. In FIG. 4A, the ion beam path 40 sweeps through the center of the wafer 41, resulting in a trace 42 having the longest width when displayed on the beam map. This trace 42 is commonly referred to as the outmost edge, and corresponds to the outmost trace 21 shown on the composite beam map in FIG. 2. Referring to FIG. 4B, the beam path 43 across the wafer 41 does not pass through the center of the wafer and, thus, has a shorter length on the wafer 41 than the beam path 40 (FIG. 4A), resulting in a trace 44 as shown in FIG. 4B. As the ion beam scans nearer to the edge of the wafer 41 as in beam path 45, a narrower trace 46 is displayed as shown in FIG. 4C. When the ion beam 47 finally reaches the edge of the wafer 41, the resultant trace 48 has a narrowest width. The traces 44, 46, and 48 described above are commonly referred to as minors, which correspond to the traces under the outmost edge 21 on the composite beam map as shown in FIG. 2A.
The beam map display is conventionally used as part of a supervision system in the ion implantation process to ensure that the implantation process parameters such as recipe, acceleration voltage, and ion dosage are within tolerances and the ions are uniformly implanted on the wafer. In order to keep the implantation process in proper adjustment, the operator needs to continuously observe the beam map display for the occurrence of an abnormal beam map (indicating the implantation process is out of adjustment) and take corrective measures to prevent further damage to the wafer. These corrective measures include adjusting the implantation system parameters; e.g., the X-plate and Y-plate voltage, or even shutting down the implanter. FIG. 2B shows a simplified schematic of a pair of conventional adjustment sticks. These sticks are used to adjust horizontal (X) plate voltage and vertical (Y) plate voltage of the beam scan by an experienced expert or operator. Unfortunately, the analysis of the ion beam display has no standard criterion. This analysis largely depends on the accumulated experience of the operator, which can result in unnecessary or improper adjustment. Further, this manual adjustment by the operator is usually too slow to prevent defective implantation of the wafer, resulting in the failure of the wafer during quality testing. Other approaches may include using a video camera to monitoring the beam map display, using techniques disclosed by Banks in "Signal Processing, Image processing and Pattern Recognition", Prentice Hall, 1990. Complex two-dimensional image processing techniques are used to try to recognize abnormal ion beam maps. However, this approach tends to be impractical due to the complexity of the two-dimensional image processing, which slows down the recognition process so that a relatively large number of faulty implantations occur before abnormal beam scanning is recognized. Thus, a need has arisen for a low cost supervision system that can quickly, automatically and accurately identify symptoms of abnormal ion beam scanning to maintain the quality of the ion implantation process in real time.